(1) Field of the Invention
The present invention pertains to the field of disk drives. More particularly, the present invention pertains to apparatus and method that utilize a head assembly to read and/or write information from/to the disk drive.
(2) Background Information
One of the key components for certain electrical devices is a place to store data thereon and read data therefrom. For example, compact disk players read data, such as music, from a plastic disk. Another example is a VCR which reads data (video information) from a tape. Computer systems are endowed with devices that may store thereon and from which it may be read therefrom large amounts of data. Computer systems employ a variety of storage devices to store data. One of these devices is a disk drive, which may also be a direct access storage device. FIG. 1 shows a disk 100 onto which data is stored in concentric circular tracks 102. A first track 116 is the closest to a spindle 106.
A disk drive 100, or direct access storage device, includes several disks 104 which look similar to records used on a record player, or compact disks which are used in a C.D. player. The disks are stacked on spindle 106, much like several records waiting to be played. In a disk drive, however, the disks are mounted into the spindle and spaced apart so that the separate disks do not touch each other.
Information is recorded on a surface 110 of each disk. The surface 110 of each disk 104 is uniform in appearance. However, in actuality, each of the surfaces is divided into portions where data is stored. There are a number of tracks 102 disposed in concentric circles like rings on a tree. Each track in a disk drive is further subdivided into a number of sectors of which sectors 117, 118, 119, and 120 are shown in the Figure. Each sector is essentially just one section of the circumferential track.
The recorded information is divided into sectors. Servo information is recorded in radially continuous narrow wedges (not shown) between sectors. The information in the servo sections includes: track number; sector number; and tracking information.
Storage of data on a magnetic disk entails magnetizing portions of the disk in a pattern which represents the data. To magnetize the disk, a small ceramic block which contains a magnetic transducer known as a "write element" is passed over the surface of the disk. More specifically, the write element is flown at a height of approximately six microinches of an inch from the surface of the disk over the track. The write element is energized to various states, causing the track below to be magnetized to represent the data to be stored.
To retrieve data stored on a magnetic disk, a read element is flown over the disk. The magnetized portions of the disk provide a signal to the read element. By looking at output from the read element, data can be reconstructed and then be used by the computer system.
Like a record, both sides of a disk are generally used to store data or other information necessary for the operation of the disk drive. Since disks are held in a stack and are spaced apart from one another, for both the top and the bottom surface of each disk in the stack of disks there is a corresponding read element and write element. This would be comparable to a stereo that would play both sides of a record at once. Each side has a stylus which may play the particular side of the record.
There are two types of disk drives: rotary and linear. Rotary disk drives have a tone arm that rotates much like a record player. The tone arm of a rotary disk drive, termed actuator arm, holds all the transducers or read/write elements--one head for each surface of each disk supported in a structure that looks like a comb. Like a tone arm, the actuator arms rotate so that the read element and write element attached to the actuator arm can be moved to locations over various tracks on the disk. In this way, the write element can be used to magnetize the surface of the disk at one of several tracks locations in a pattern representing the data. The read element is used to detect the magnetized pattern on one of the tracks of a disk. For example, the needed data may be stored on two different tracks on one particular disks, so to read the magnetic representations of the data, the actuator arm is rotated from one track to another track.
Before writing or reading, the magnetic head must be positioned in close proximity of or above the correct track. This is accomplished by mounting the head on an arm that moves radially, or in an are to the section of the disk allocated to the write/read operation. In rigid disk drives, many head assemblies and support arms are mounted on one common carriage that is moved as one unit. This is shown in FIG. 2 where a four-platter disk drive 202 is shown with a head carriage 204 moved back and forth by linear voice-coil motor (VCM) 216 that may be identical to the "motor" in a loudspeaker. In linear disk drives, typically, for each disk (platter) 210 there is a double positioning arm 212 that positions the head onto a specific track by virtue of linear movement of the head carriage 204, to which the positioning arm 212 is mounted. FIG. 3 shows a top view of a disk 302 coupled to a linear VCM 304. A linear actuator is disposed inside carriage 306. The actuator is coupled to the positioning arm 308 that has a head mounted at a free end thereof. The actuator causes the head to shift to new track positions.
Rigid disk drives demand fast and precise positioning of the head. At low track densities, it is relatively easy to position the head; at higher densities, it is necessary to use a track-following technique, by using a closed-loop servo. Overall performance of a disk drive is measured by its seek time, which is the average time it takes for a head to move from one track to another (including time to settle over the track), plus the latency, which is the time for one revolution (some may say that the average latency represents the time for one-half revolution).
The access time (to a track) is in the approximate range of 10-30 milliseconds (ms). To position a head accurately, full-forward power of the actuator is applied for half the time required. Then reverse power is applied until the actuator comes to zero velocity. When the process is done, if the control system functions properly, it may position the head precisely on target. This is what servo engineers call a "bang-bang" control.
The servo system keeps track of where the head is. This is typically done with a difference counter that contains the number of tracks to the target. The difference counter is updated as tracks are crossed. When the head has been positioned over the right track, the next task for the disk controller's servo system is to keep the head on track irrespective of mechanical vibrations, aerodynamics disturbances, or changes due to variations in the temperature.
At high-track densities, a direct position feedback from the head itself is necessary to correct for any mistracking. Track misregistration (TMR) may be caused by several items: spindle run-out; resonances and disk flutter; thermal track shift; head settling; actuator interaction; and improper servo writing. A disk drive's TMR is therefore a summation of all the head position errors summed up over a period of time.
Stringent requirements are posed on the head and the mechanism that controls the head such as track-to-track access time, accurate positioning of the head by the servo-mechanism, and reading by the head from the storage device when the head has stopped moving completely. It is desirable to provide a disk-head assembly with improved performance, including decreased track-to-track access times, where reading of data may be performed even when the head has not necessarily been stopped from moving completely. It is also desirable to compensate for effects such as spindle run-out, resonance and thermal effects without having to move the heads as much or as rapidly as current systems require.